The present invention relates to amplifiers for integrated circuits, particularly to variable-gain amplifiers.
Variable-gain amplifiers are useful in applications such as RF receivers where a fixed output voltage level is desirable, but the signal strength of an input signal varies. If a received signal is weak, a variable-gain amplifier should gain the signal to the desired output level, when the received signal is strong, the gain should be lowered.
The gain is varied by a control voltage such that a change in control voltage results in a change in the amplifier gain. Unfortunately, this change in gain as a function of control voltage is typically nonlinear and temperature dependent. This makes the setting of the control voltage more difficult because a step in the Digital-to-Analog converter that steers the Variable-Gain-Amplifier does not correspond to a fixed dB value. Also, the resulting gain is a function of temperature, which results in a gain drift.
Any attempt to improve the linearity and temperature drift of the amplifier gain is complicated by the trend towards lower operating voltages. This trend has been driven by a desire for lower power supply dissipation, longer battery life in mobile devices, and the use of smaller geometry devices. Voltage supplies, while plus and minus 15 volts many years ago, were lowered to 5 volts, then 3.3 volts, and are now at 1.8 volts. These voltages are certain to be reduced again in the future. Designing high performance circuits that can operate at these lower voltages requires innovation, particularly in circuits made using a bipolar process, since transistor base-to-emitter voltages have not correspondingly reduced along with supply voltage.
Thus, what is needed is a variable-gain amplifier that has a gain versus control voltage response that is linear in dB and is temperature independent and operates properly at these lower voltages.